1. Field of the Invention
This invention relates to a photosensitive resin composition. More particularly, it relates to a photosensitive resin composition which may be used conveniently as a negative type photoresist for excimer laser lithography.
2. Description of the Prior Art
Recently, in keeping with the progress of VLSIs or ULSIs and with the miniaturization of the design rule of semiconductor devices, investigations for increasing the number of apertures of the light exposure apparatus, shortening the exposure light wavelengths and developing improved photoresist materials, have been conducted briskly in the field of photolithography. Above all, excimer laser lithography employing an excimer laser light source, such as KrF excimer laser light (248 nm), is attracting attention as the exposure light source in place of the conventional light source, such as g-line (436 nm) or i-line (365 nm) of the high pressure mercury lamps, as the technology for facilitated realization of a higher resolution in photo lithography.
Meanwhile, with the excimer laser lithography, it is difficult to make direct application of the novolak-based positive type photoresist which has been customarily employed for g-line or i-line exposures. The reason is that, since the novolak resin as the base resin and the aromatic rings of naphthoquinone diazide compounds added as the photo sensitive material exhibit high absorption in the wavelength range of the excimer laser light, sensitivity falls short, while transmissivity of the exposure light is also markedly lowered, thus resulting in a tapered cross-sectional shape of the photoresist pattern.
Under this situation, a demand has been raised for a photoresist material capable of realizing high sensitivity and high resolution in the excimer laser wavelength region. Recently, a so-called chemically amplified resist is attracting attention as this type of photoresist material. The chemically amplified resist is a photoresist of the type in which an acid catalyst is generated by an optical reaction from a photoreactive acid catalyst generator, referred to hereinafter as photoreactive acid generator, such as onium salt or polyhalogenide, and in which the resist reaction such as polymerization, cross-linking or functional group conversion, is proceeded by carrying out heat treatment in the presence of this acid catalyst, thereby causing changes in the rate of dissolution. The term "chemically amplified" is based on the fact that, since the change in the rate of dissolution is the catalytic reaction, the apparent quantum yield is high enough to produce a high sensitivity resist.
The chemically amplified resist is classified into a positive type resist and a negative type resist, depending on the type of the resist reaction, and into a two-component system and a three-component system, depending on the number of the basic components of the resist. Typical of this type of the resist is a negative type three-component system resist in which the novolak resin is used as the base resin, p, p'-dichlorodiphenyltrichloroethane (DDT) is used as the photoreactive acid generator and hexamethoxymethylmelamine is used as the acid-crosslinking agent.
However, the novolak resin suffers from a disadvantage that, since it exhibits higher absorption in the excimer laser wavelength region, as mentioned above, the resist film cannot be increased in thickness and thus the resin can not be applied to a monolayer resist process without considerable difficulties. For example, the resist film thickness capable of coping with the fine design rule of the order of 0.35 .mu.m in terms of the gate electrode widths of MOS transistors is 0.5 .mu.m at the maximum.
The present inventors have previously proposed a negative type two-component resist in which polyvinyl phenol having superior light transmitting properties is used as the base resin in place of the novolak resin and bis(polyhalomethyl) benzene is used as the photoreactive acid generator.
However, the above mentioned conventional chemically amplified resist leaves much to be desired.
For example, although it is possible with the negative type two-component system resist, previously proposed by the present inventors, to resolve the line and space widths each 0.3 .mu.m highly satisfactorily when KrF excimer laser light exposure is made with the resist film thickness of 1 .mu.m, a light exposure energy not less than 200 mJ/cm.sup.2 is required, thus leaving much room for improvement in sensitivity in the excimer laser lithography.
On the other hand, with the above mentioned negative type three-component system resist, the KrF excimer laser light energy of 60 to 80 J/cm.sup.2 is required for resolution for the assumed case of the heat treatment at 110.degree. C. continuing for 90 seconds, so that further improvement in sensitivity is required of this resist for practical applications.